Many inks and particularly those used in thermal ink jet printing include a colorant and a liquid which is typically an aqueous liquid vehicle. Some thermal ink jet inks also include a low vapor pressure solvent. When a substrate or a sheet of paper is printed with ink jet ink, the ink is deposited on the substrate to form an image in the form of text and/or graphics. Once deposited, the liquid is removed from the ink and paper to fix the ink to the substrate. The amount of liquid to be removed, of course, varies with the amount of ink deposited on the substrate. If a sheet is covered with 10% printing, as in text only printing, the amount of liquid to be removed is quite small. If the sheet is covered with 90% printing, however, as when a graphic image is printed, the amount of liquid to be removed is substantially more and can cause image defects and paper deformation if not removed very rapidly.
Liquid can be removed from the ink and printed substrate by a number of methods. One simple method is natural air drying in which the liquid component of the ink deposited on the substrate is allowed to evaporate without mechanical assistance resulting in natural drying. Another method is to send the printed substrate through a dryer to evaporate the liquid. In some cases a special paper is used in which the liquid is absorbed by a thin coating of absorptive material deposited on the surface of the paper. Blotting of the printed substrate is also known.
In the case of natural drying, almost 100 percent of the liquid is absorbed into the paper and is then, over a long period of time, evaporated naturally. The absorption and desorption of water into and out of the paper, however, has some undesirable side effects, such as long drying time, strike through, feathering at edges of the printed image, paper curl and paper cockle. In the case of paper cockle, the absorption and desorption of the water relaxes the internal stresses of the paper and results in deformations known as cockle. Cockle is also a function of the amount of liquid deposited per unit area. Less printing on a page has less potential to develop cockle due to the smaller amount of liquid. More printing on a page has more cockle potential due to a higher amount of liquid per unit area. Cockle can also be induced by heating of the paper, which results in stress relief.
Ink compositions also have an effect on the drying rates and drying efficiency. For example, highly absorptive (fast drying) inks while requiring less ink to be removed by a dryer are prone to image quality defects such as feathering, raggedness, and strike through. On the other hand, slightly absorbtive inks require more power from a dryer to dry since more ink requires evaporation.
The rate at which the image is dried is also critical for controlling the print quality. A slow drying rate can achieve ink permanence or drying effectiveness but also can result in image quality defects such as excessive image feathering or strike through. Additionally, a slow drying rate can result in image offset (ink from one sheet of paper is transferred to another sheet of paper because the ink has not dried completely), smear and spreading from contact with exit rolls, baffles and output stacking of the individual sheets. A very fast drying rate can result in image mottle and image spatter.
Drying rates are particularly critical when substrates are printed at high rates of speeds. Not only must image deformations and paper deformations be controlled, but the drying times must be short due to the high printing rates to ensure no offset at exit rolls.
A dryer must achieve image fixing (no offset/smear) and good image quality to reduce or prevent image disturbance, distortion, feathering and strike through. In addition the dryer must preferably reduce or eliminate cockle and curl. Besides the slow speed of conventional dryers, many dryers produce uneven drying rates resulting in uneven drying patterns. To shorten drying times, infrared drying techniques have been adopted. This method can, however, cause browning of paper during paper jams due to the elevated temperatures produced by the infrared heat.
In U.S. Pat. No. 3,584,389 to Hilton et al., a method and apparatus for drying printing ink on paper in which both microwave and infra-red radiation used for heating is described. A web of printed paper on which the print is to be dried passes through two serpentine slotted waveguides in succession, and thereafter passes under three infra-red heaters.
U.S. Pat. No. 3,617,953 to Kingma et al. describes a microwave impedance matching system for matching a microwave input waveguide to a microwave output waveguide. A first and second electromechanical phase shifter are moved transversely in waveguide sections to produce varying amount of differential phase shift.
U.S. Pat. No. 3,672,066 to Stephansen discloses a microwave drying apparatus which includes a serpentine waveguide The device includes two opposed air cushions which force a web of material passing through the device to stay midway between the opposite waveguides.
U.S. Pat. No. 3,739,130 to White, discloses a multicavity microwave applicator for uniformly treating a moving web of material with microwave energy. The applicator is formed of two separate sets of cavity resonators which are intermeshed with one another so that the resonators of one set are alternated with the resonators of the second set in a side-by-side relationship.
U.S. Pat. No. 3,783,414 to Klein et al. describes a termination for a transmission line or waveguide of small weight and size capable of absorbing high levels of power and capable of achieving a VSWR in the order of 1.05 to 1.20 over 10-20 percent frequency bands.
U.S. Pat. No. 3,796,973 to Klein describes a termination for transmitting or absorbing a signal transmitted through a transmission line or waveguide.
U.S. Pat. No. 4,234,775 to Wolfberg et al. discloses a device which has a serpentine waveguide and uses microwave energy to remove moisture from a moving web. The microwave energy takes the form of standing waves which are purposefully disrupted to cause the peaks of the standing waves to continuously oscillate along the various sections of the waveguide, resulting in a more uniform application of the microwave energy across the width the web.
U.S. Pat. No. 4,286,135 to Green et al. describes a waveguide isolator having microwave ferrite bars to reduce energy reflected into the microwave source. A blower fan draws air past the microwave source and through a waveguide to provide cooling.
U.S. Pat. No. 4,332,091 to Bensussan et al. describes a microwave drying device intended for drying grains having a phase shifter allowing the phase to be adjusted to obtain maximum efficiency of the device.
U.S. Pat. No. 4,352,691 to Owatari et al. describes liquid ink compositions including a water-soluble dye, an alkali material, at least one wetting agent and water suited for ink jet type printers. Ink drying time is about five seconds.
U.S. Pat. No. 4,469,026 to Irwin describes a method and apparatus for drying ink printed on print media. Drying is controlled according to print parameters such as print data density, characteristics of the ink, and ambient humidity. The dryer may be a conventional hot roll, a hot platen, a lamp or a microwave dryer.
U.S. Pat. No. 4,482,239 to Hosono et al. describes an electrophotographic copying machine which visualizes electrostatic latent images with developer fixed by microwave radiation. The developer is a colored developing powder composed of thermoplastic resin having a high dielectric constant and magnetic powder having magnetic loss.
U.S. Pat. No. 4,754,238 to Schuller et al. describes a microwave absorber including a hollow body consisting of microwave-absorbing material which is arranged in a housing. At least one inlet and one outlet are provided for a gaseous cooling fluid which streams through the container to carry away heat produced by microwave energy which has been absorbed by the absorbing body.
U.S. Pat. No. 4,970,528 to Beaufort et al. describes a method for uniformly drying ink on paper from an ink jet printer. While paper is transferred from an input paper supply tray to an output paper collection tray, the paper receives the uniform heat flux from an infrared bulb which is located on the axis of symmetry for the paper transport path. The per page processing speed for this apparatus was increased from 2 minutes per page using no dryer at all, to 13 seconds per page using the described uniform dryer.
U.S. Pat. No. 5,079,507 to Ishida et al. describes an automatic impedance adjusting apparatus for adjusting an impedance seen looking toward a microwave load. A cooling air outlet exhausts cooling air into a circular waveguide.
U.S. Pat. No. 5,207,824 to Moffatt et al. describes ink formulations for control of paper cockle in thermal ink jet printing. The ink contains the components of water, dye, and a low vapor pressure solvent, which contains an organic compound or anti-cockle agent.
U.S. Pat. No. 5,214,442 to Roller describes an adaptive dryer to minimize heating power requirements of a printer by determining mass-area coverage of ink on a page prior to drying.
U.S. Pat. No. 5,220,346 to Carreira et al. describes a printing process using an ink composition comprising an aqueous liquid vehicle, a colorant and an ionic compound at least partially ionizable in the liquid vehicle applied to a substrate in an imagewise fashion and subsequently exposed to microwave radiation to dry the images on the substrate.
British Patent Specification No. 1,050,493 to Hilton describes microwave heating and/or drying of sheet material, for example paper in order to dry ink which has been applied by a printing process. The apparatus comprises a plurality of waveguide sections provided with slots in the sides thereof through which a sheet of material can be passed for drying. The waveguide sections are arranged in a serpentine manner. A microwave source is attached to one end of the waveguide and a load is attached to the other end of the waveguide.
Japanese Laid Open Publication 107,490 to Yamaguishi describes an apparatus to dry ink printed out by means of an ink jet printer by utilizing a microwave.
European Patent Application Publication No. 538 071-A2 to Yasuhiko et al. describes an ink jet recording apparatus having a low temperature heater and a high temperature heater to dry the ink. The low temperature heater heats the recording sheet and the high temperature heater heats the ink recorded on the recording sheet to a temperature at which the ink is dried and fixed.
Xerox Disclosure Journal, Volume 7, Number 6, November/December 1982, pages 373 to 375 describes an electrostatic dryer for ink jet printers.